1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a bonding pad structure of a semiconductor device and a method of bonding a bonding wire to a bonding pad.
2. Description of the Related Art
A semiconductor substrate is mounted on a printed circuit board, wherein bonding pads of the semiconductor substrate are bonded through bonding wires to lead frames of the printed circuit board. In view of the LSI manufacturing processes, it is preferable that the bonding pads are made of the same metal as internal wirings or interconnections of the semiconductor substrate. For example, Al or Alxe2x80x94Cu alloy are available. In view of the cost performance, the bonding wires are usually made of Al or Alxe2x80x94Si alloy.
Both the bonding pads and the bonding wires made of the Al-based materials provide an advantage in reduction of the manufacturing cost. If, however, the semiconductor device is applied to the high out device, then the bonding wires receive a high density current, resulting in a possibility of disconnection of the bonding wires. In order to avoid this problem, it is effective to increase a sectioned area or a diameter of the bonding wires. This increase in sectioned area or diameter of the bonding wires is, however, unsuitable for the increase in the density of the integration of the semiconductor device.
Alternatively, Au or Au alloy is also available for the bonding pads and the bonding wires. These Au-base materials are superior in anti-corrosion, and also lower in resistivity than Al, for which reason the Au-based bonding wires have a relatively high reliability without disconnection due to the high density current application. A disadvantage with use of the Au-base materials is that the Au-based bonding pads results in the increase of the manufacturing cost.
To avoid the increase of the manufacturing cost, it was proposed that the Al-base material is used for the bonding pads, and the Au-base material is used for the bonding wires. Since the Au-based bonding wires have a low resistance, it is possible to reduce the diameter or the sectioned area of the wires to take a large inductance. This means it easy to take an impedance matching.
The followings are advantages and disadvantages of the possible combinations in use of the Al and Au for the bonding pads and the bonding wires.
If Al is used for the bonding pads and Au is used for the bonding wires, then the bonding is carried out by an Alxe2x80x94Au alloy bonding, thereby providing advantages in a low manufacturing cost and a good impedance matching due to reduction in diameter of the bonding wires and an enlarged inductance. A disadvantage is that a bonding resistance is increased at a high temperature.
If Au is used for both the bonding pads and the bonding wires, then the bonding is carried out by a thermal compression bonding, thereby providing an advantage in a high thermal stability and also a disadvantage in high manufacturing cost.
If Al is used for both the bonding pads and the bonding wires, then the bonding is carried out by a thermal compression bonding, thereby providing advantages in a high thermal stability and low manufacturing cost and also a disadvantage in a large diameter of the bonding wires in view of avoiding the disconnection of the high density current application,
The combined use of the Al-base material for the bonding pads and the Au-base material for the bonding wires provides a disadvantage in that a brittle Alxe2x80x94Au alloy is formed at a junction interface of the Al-based bonding pads and the Au-based bonding wires at a relatively low temperature, for example, in the range of 150-200xc2x0 C. Thus, during a high temperature condition caused by high output driving of the semiconductor device, the brittle Alxe2x80x94Au alloy is formed at the junction interface, whereby a resistance of the junction between the Al-based bonding pads and the Au-based bonding wires increases and the bonding strength decreases.
The temperature in the normal use of the semiconductor device is only about 80-90xc2x0 C. Under this low temperature condition, the brittle Alxe2x80x94Au alloy is not grown after the Al-based bonding pads and the Au-based bonding wires have been bonded. The temperature in the high output use of the semiconductor device is sometimes over 150xc2x0 C. Under this high temperature condition, the brittle Alxe2x80x94Au alloy is grown up even after the Al-based bonding pads and the Au-based bonding wires have been bonded. As a result, the junction resistance is remarkably increased. The bonding strength is weaken. This problem will further be described in detail.
FIG. 1 is a side view of a conventional wire-bonding structure between a bonding pad on a semiconductor device and a lead frame on a printed circuit board. FIG. 2 is a fragmentary cross sectional view of a bonding interface between a bonding wire and a bonding pad immediately after the bonding process and before the high output driving of the semiconductor device. FIG. 3 is a fragmentary cross sectional view of a bonding interface between a bonding wire and a bonding pad after the high output driving of the semiconductor device.
As illustrated in FIG. 1, bonding pads 2 having a thickness of about 1 micrometer are provided on a surface of a semiconductor device 1 which is placed on a printed circuit board 10. The bonding pads 2 are bonded through bonding wires 3 to leads frames 4 of the printed circuit board 10. The bonding pads 2 are made of an Al-base material. The bonding wire 3 is made of an Au-base material.
As shown in FIG. 2, the bonding wire 3 has a ball 6 which is bonded with the bonding pad 2 by a thermal compression bonding. The bonding wire 3 is further bonded to the lead frame 4 by use of a bonding tool 5. The ball 6 of the bonding wire 3 digs into the bonding pad 2 by the thermal compression bonding, whereby an Auxe2x80x94Al alloy layer 10 is formed on a bonding interface between the ball 6 of the bonding wire 3 and the bonding pad 2.
FIG. 3 illustrates the grown Auxe2x80x94Al alloy layer 10 on the bonding interface after the high output driving of the semiconductor device at a high temperature for a long time. Under the high temperature condition, the Auxe2x80x94Al alloy layer 10 is grown up to increase the thickness uniformly. The Auxe2x80x94Al alloy layer 10 is higher in resistivity than the bonding pad 2 and the bonding wire 3. The current flows through the Auxe2x80x94Al alloy layer 10 in the thickness direction, for which reasons the increase in thickness of the Auxe2x80x94Al alloy layer 10 increases the resistance, to which the current senses. Namely, the increase in thickness of the Auxe2x80x94Al alloy layer 10 increases the bonding resistance between the bonding pad 2 and the bonding wire 3. Further, a bonding deterioration also increases the bonding resistance between the bonding pad 2 and the bonding wire 3.
In the above circumstances, the development of a novel bonding structure between a bonding pad and a bonding wire free from the above problems is desirable.
Accordingly, it is an object of the present invention to provide a novel bonding structure between a bonding pad and a bonding wire, wherein the bonding structure makes a semiconductor device free from the above problems.
It is a further object of the present invention to provide a novel bonding structure between a bonding pad and a bonding wire, which suppresses an increase in resistance of a boning part between the bonding pad and the bonding wire under a high temperature condition.
It is a still further object of the present invention to provide a novel bonding structure between a bonding pad and a bonding wire, which suppresses a weakening a bonding strength between the bonding pad and the bonding wire under a high temperature condition.
It is yet a further object of the present invention to provide a novel bonding pad bonded with a bonding wire, wherein the bonding pad makes a semiconductor device free from the above problems.
It is a further object of the present invention to provide a novel bonding pad bonded with a bonding wire, wherein the bonding pad suppresses an increase in resistance of a boning part between the bonding pad and the bonding wire under a high temperature condition.
It is a still further object of the present invention to provide a novel bonding pad bonded with a bonding wire, wherein the bonding pad suppresses a weakening a bonding strength between the bonding pad and the bonding wire under a high temperature condition.
It is yet a further object of the present invention to provide a novel semiconductor device having an improved bonding structure between a bonding pad and a bonding wire, which suppresses an increase in resistance of a boning part between the bonding pad and the bonding wire under a high temperature condition.
It is a still further object of the present invention to provide a novel semiconductor device having an improved bonding structure between a bonding pad and a bonding wire, which suppresses a weakening a bonding strength between the bonding pad and the bonding wire under a high temperature condition.
It is yet a further object of the present invention to provide a novel semiconductor device having an improved bonding pad bonded with a bonding wire, wherein the bonding pad makes a semiconductor device free from the above problems.
It is a still further object of the present invention to provide a novel method of bonding a bonding pad and a bonding wire, wherein the bonding method makes a semiconductor device free from the above problems.
It is a further object of the present invention to provide a novel method of bonding a bonding pad and a bonding wire, which suppresses an increase in resistance of a boning part between the bonding pad and the bonding wire under a high temperature condition.
It is a still further object of the present invention to provide a novel method of bonding a bonding pad and a bonding wire, which suppresses a weakening a bonding strength between the bonding pad and the bonding wire under a high temperature condition.
The present invention provides a bonding structure between a bonding pad and a bonding portion of a bonding wire made of an Au-base material, wherein said bonding pad further comprises: a base layer; at least a barrier layer overlying said base layer; and a bonding layer overlying said at least barrier layer, said bonding layer including an Al-base material, and wherein said bonding portion of said bonding wire is buried in said bonding layer, and an Auxe2x80x94Al alloy layer extends on an interface between said bonding portion and said bonding layer, and a bottom of said Auxe2x80x94Al alloy layer is in contact with or adjacent to an upper surface of said barrier layer.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.